The present invention relates to a mobile telecommunication system; and, more particularly, to a mobile telecommunication system and a method for performing a handoff from an asynchronous base station such as W-CDMA system to a synchronous base station such as 2G system based on IS-95 and 3G system based on IS-2000.
Generally, a mobile telecommunication system includes a synchronous communication system and an asynchronous communication system. The synchronous communication system operates in synchronization with a master clock provided by a global positioning system (GPS), and the asynchronous communication system operates without regard to the master clock. Furthermore, the synchronous communication system performs a data interface or a data transmission on the basis of interim standard (IS) protocols such as IS-95 and IS-41, and the asynchronous communication system performs a data interface or a data transmission on the basis of a universal mobile telecommunication system (UMTS) terrestrial radio access (UTRA) standard.
As the number of users is rapidly increasing and a demand on multi-media is rising high, a mobile telecommunication system is required which is capable of transmitting and receiving various types of information. For satisfying that requirement, an international telecommunication union (ITU) has suggested an international mobile telecommunication-2000 (IMT-2000) system as a next-generation mobile telecommunication system.
In network developments of the IMT-2000 system, there are four network interface architectures as follows:
1) dual-mode mobile station operating in a synchronous modexe2x80x94synchronous radio networkxe2x80x94synchronous core network (ANSI-41 network);
2) dual-mode mobile station operating a synchronous modexe2x80x94synchronous radio networkxe2x80x94asynchronous core network (GSM-MAP network);
3) dual-mode mobile station operating in an asynchronous modexe2x80x94asynchronous radio networkxe2x80x94asynchronous core network (GSM-MAP network); and
4) dual-mode mobile station operating in an asynchronous modexe2x80x94asynchronous radio networkxe2x80x94synchronous core network (ANSI-41 network).
FIGS. 1A to 1D are views showing network interface architectures of IMT-2000 system. In the drawings, the dual-mode mobile stations 10 are a mobile station that can selectively interface with the synchronous communication system and the asynchronous communication system according to a type of the radio networks.
An Air-interface between the dual-mode mobile station operating in the asynchronous mode and the asynchronous radio network employs an asynchronous communication Air-interface protocol, and an A-interface between the asynchronous radio network and the synchronous core network employs a 3 generation interoperability specification (3G-IOS) protocol.
In case of FIG. 1D, however, there are differences between the Air-interface and the A-interface in a call control, a procedure of a handoff operation, kinds of messages and parameters, and the like. Therefore, an effective call control and handoff method is needed.
The handoff is an operation for assigning a new communication channel to a mobile station when the mobile station moves from one specific cell to another cell neighboring to the specific cell. In handoff techniques, there are a hard handoff and a soft handoff.
When a mobile station moves from one cell to another cell, the hard handoff is achieved by breaking a communication channel of one base station and then making a call connection to a communication channel of another base station. At this time, there may occur a phenomenon of a breaking of communication.
When a mobile station moves from one cell to another cell, the soft handoff is achieved by making a call connection to a communication channel of another base station and then breaking a communication channel of one base station.
When the mobile station communicating with an asynchronous base station moves to an area where there are no asynchronous base stations capable of providing a communication service to the mobile station, it is necessary to perform a handoff from a current asynchronous base station to a synchronous base station neighboring to the current asynchronous base station.
For performing the handoff of an asynchronous base station to neighboring synchronous base station, a mobile station should obtain information associated with the synchronous base station, such as a pilot offset of the neighboring synchronous base station, a superframe timing of a Sync channel, a long code state, and the like. Also, in order to minimize a phenomenon of an undesirable breaking of communication, the mobile station should obtain the information before a compressed mode.
The pilot channel of entire synchronous base stations employs one short code having a length of 215 chips (26.667 ms), and each synchronous base station are identified according to corresponding pilot offset of the pilot channel. Therefore, for searching for the pilot channel during the compressed mode, a total period of 26.677 ms should be searched, thereby causing a waste of time.
Additionally, for obtaining the pilot offset and the long code state of the traffic channel, the Sync channel needs to be decoded. Since the superframe of the Sync channel has a period of 80 ms, however, it takes much time to perform the handoff from the asynchronous base station to the synchronous base station.
Furthermore, in case where the mobile station does not obtain the information associated with the neighboring synchronous base station, the mobile station cannot immediately perform a power monitoring operation when the mobile station receives a power monitoring command from the asynchronous base station. As a result, the mobile station performs an initial cell searching operation in order to obtain information associated with neighboring synchronous base stations. However, it takes much time to perform the initial cell searching operation, thereby causing a phenomenon of an undesirable breaking of communication.
It is, therefore, an object of the present invention to provide a mobile telecommunication system for performing a handoff from an asynchronous communication system to a synchronous communication system by using information associated with a neighboring synchronous base station.
It is another object of the present invention to provide a method for performing a handoff from the asynchronous communication system to the synchronous communication system by using information associated with a neighboring synchronous base station.
In accordance with an aspect of the present invention, there is provided a mobile telecommunication system for performing a handoff from an asynchronous communication system to a synchronous communication system, comprising: at least one asynchronous base station; at least one synchronous base station neighboring to the asynchronous base station, wherein the synchronous base station transmits channel signals; at least one dual-mode mobile station for selectively communicating with the synchronous base station and the asynchronous base station; and a reception means, coupled to the asynchronous base station, for receiving and decoding the channel signals to transmit decoded signals to the asynchronous base station, wherein the asynchronous base station transmits the decoded signals to the dual-mode mobile station, thereby performing a handoff operation.
In accordance with another aspect of the present invention, there is provided a method for performing a handoff from an asynchronous communication system to a synchronous communication system in a mobile telecommunication system, wherein the mobile telecommunication system includes an asynchronous base station, a synchronous base station neighboring to the asynchronous base station, the synchronous base station transmitting channel signals, a dual-mode mobile station for selectively communicating with the synchronous base station and the asynchronous base station, and a reception means, coupled to the asynchronous base station, for receiving and decoding the channel signals to transmit decoded signals to the asynchronous base station, the method comprising the steps of: a) receiving the channel signals; b) communicating with the dual-mode mobile station operating in an asynchronous mode; c) searching neighboring asynchronous base stations to be handed off; d) if there is no neighboring asynchronous base station to be handed off, transmitting a power monitoring command to the dual-mode mobile station; e) transmitting decoded signals to the dual-mode mobile station; f) performing a power monitoring operation and report monitored result to the asynchronous base station; and g) performing a handoff to the synchronous base station.